1. Field of the Invention
The present invention relates to multi-layered diffraction optical elements comprised of a low refractive index and high dispersion material and a high refractive index and low dispersion material which are laminated with no space between thereof.
2. Description of the Related Art
Heretofore, in general, a diffractive optical system using diffraction of light has combined lenses comprised of glass materials different in dispersion characteristics, thereby reducing chromatic aberration.
For example, in objective lenses such as a telescope, a glass material small in dispersion has been taken as a plus lens and a glass material large in dispersion as a negative lens, and by the combined use of these lenses, chromatic aberration appearing on an optical axis has been corrected. However, when the configuration and the number of lenses are restricted or when the glass materials to be used are limited, it has been often found difficult to sufficiently correct the chromatic aberration.
A non-Patent Document 1 (A. D. Kathman and S. K. Pitalo, “Binary Optics in Lens Design”, International Lens Design Conference, 1990, SPIE Vol. 1354, p 297-309) discloses that a combined use of the diffraction optical element having a diffraction face and the diffraction optical element having a diffraction grating can reduce the chromatic aberration with a small number of lenses used.
This takes advantage of a physical phenomenon in which a refracting face and a diffracting face as the optical element are reversed in the generating direction of the aberration for the light of a reference wavelength. By changing a cycle of the diffraction grating continuously formed in the refraction optical element, a characteristic equivalent to an aspherical lens can be developed.
However, a piece of light entering the diffraction optical element is divided into a plurality of lights of each degree of order by a diffraction action. At this time, the diffraction light other than a design degree is image-formed at a place other than the light of the design degree, thereby to become a generating cause of flare.
U.S. Pat. No. 5,847,877 discloses that a refractive index dispersion of each optical element and a configuration of grating formed on a boundary surface of the optical element are optimized, so that a high diffraction efficiency is realized in a wide range of wavelengths. A light flux of the usable wavelength range is focused on a specific degree of order (hereinafter, referred to as a design degree), thereby to hold down the intensity of the diffraction light of other diffraction degree of orders and prevent flare from occurring.
U.S. Pat. No. 5,847,877 discloses that, in order to obtain a configuration having high diffraction efficiency in a wide range of wavelengths, a diffraction optical element formed by a material having relatively low refractive index dispersion and a diffraction optical element formed by a material having relatively high refractive index dispersion are combined to be used.
That is, higher a difference between the refractive index dispersions of the materials high and low in refractive index dispersion is, lower the thickness of diffraction grating of the optical element to be formed is, so that a field angel of the optical element becomes wider. Consequently, to correct the chromatic aberration with high accuracy, it is necessary to use the material having much higher (small in Abbe number) refractive index dispersion and the material having much lower (large in Abbe number) refractive index dispersion.
U.S. Pat. No. 7,031,078 discloses an optical material in which the relationship between a refraction index (nd) and an Abbe number (νd) is nd>−6.667×10−3νd+1.07, and the relationship between a secondary dispersion (θg, F) of the refractive index and the Abbe number (νd) is θg, F≦−2νd×10−3+0.59. By satisfying these formulas, refraction efficiency in the entire visible area can be improved.
The optical material in U.S. Pat. No. 7,031,078 is a composite material in which a transparent conductive metal oxide high in refractive index dispersion and showing a nature low in secondary dispersion characteristic is mixed and dispersed in binder resin as fine particles. As the transparent conductive metal oxide, a transparent conductive metal oxide such as ITO, ATO, SnO2, or ZnO is disclosed.
The embodiments of U.S. Pat. No. 7,031,078 discloses also a laminated diffraction optical element in which a diffraction optical element comprised of a material having high refraction and high dispersion and a diffraction optical element comprised of a material having low refraction and low dispersion are oppositely disposed with a space provided between thereof.
On the other hand, a demand for miniaturization of a product has been extremely increased in the optical instrument using an optical element. Thus, the development for making the thickness of the optical element as thinly as possible has been underway. Hence, being developed is not the laminated diffraction optical element in which a space exists between the diffraction optical element of a first layer and the diffraction optical element of a second layer described in the above-mentioned, but a multi-layered diffraction optical element of the type in which, no space exists. U.S. Pat. No. 6,759,471 discloses a multi-layered diffraction optical element of the type in which no space exists.
However, in the optical element described in U.S. Pat. No. 6,759,471, a combination of a low refractive index and high dispersion material in which inorganic fine particles have been dispersed with a high refraction and low dispersion glass is used. Usually, coefficient of linear expansion of an organic resin is greater than that of a glass by one or two digits.
Further, the above difference between the coefficients of linear expansion is greatly correlative to a dependency of refraction index on temperature so that the refraction index difference between the organic resin and the glass considerably changes, depending on temperature changes to decrease the diffraction efficiency.